Preheat the oven to 175°C.
Finely chop the California walnuts and toast in a dry frying pan, until golden brown, stirring often to ensure they don’t catch. Remove from the pan and leave aside to cool. Next, melt the white chocolate and butter in a heatproof bowl over a pan of simmering water. Remove the bowl and leave aside to cool.
In a large bowl sift together the flour and baking powder and set aside. Next, separate the eggs and whisk the yolks, then fold in 150g of the sugar, the salt, vanilla sugar and buttermilk.
Once combined, stir in the white chocolate and butter mixture, and the toasted California walnuts. Finally, carefully fold in the flour mix.
Line a 12-hole muffin tray with cupcake cases and divide the cake mixture equally.
Bake in preheated the oven for 20-25 minutes or until risen and golden, then leave to rest for around 5 minutes. Remove them carefully from the tray and leave aside to cool.
In a large heatproof bowl, whisk the egg whites, remaining sugar and lemon juice with an electronic hand whisk over a saucepan of simmering water until the beaten egg whites are firm and glossy. Remove from the heat and whisk again for 5-10 minutes until the mixture is cool.
Pour the frosting mixture into a piping bag with a large open nozzle and pipe icing on top of each cupcake. Let the cupcakes chill for 30 minutes so the frosting can set.
Heat the whipping cream in a small saucepan along with the coconut oil until melted. Chop the dark chocolate, then add to the cream. Once this has melted leave aside to cool. Spread the chocolate cream mixture over the chilled cupcakes and place a California walnut half on each cupcake to serve.